1. Field of the Invention
The present invention relates to a solid state image pickup apparatus for selectively reading out a plurality of sensor signals and, more particularly, to a solid state image pickup apparatus capable of eliminating unnecessary components such as variations in dark signals and drive noise.
2. Related Background Art
FIG. 1A is a schematic circuit diagram of a conventional solid-state image pickup apparatus.
Referring to FIG. 1A, signals from sensors S1 to Sn are respectively amplified by amplifiers A1 and An, and transistors T1 to Tn are sequentially turned on. A dot sequential output appears on an output line 101. The dot sequential signal is amplified by a buffer amplifier 102, and the resultant signal appears as an output signal Vout.
In the conventional image pickup apparatus described above, variations in input/output characteristics of the amplifiers A1 to An are included in the sensor signals as the dot sequential output appearing on the output line 101. As a result, steady pattern noise occurs.
FIG. 1B shows a schematic arrangement of another conventional photoelectric transducer apparatus.
Referring to FIG. 1B, signals read out from photosensors S1 to Sn are temporarily stored in storage capacitors C1 to Cn. Transistors T1 to Tn are sequentially turned on at timings of a scanning circuit SH, and the readout signals sequentially appear on an output line 101 and are output to an external device through an amplifier 102.
In the above photoelectric transducer apparatus, however, unnecessary components such as dark signals and drive noise of the photosensors are undesirably included.
Drive noise is defined as noise generated when a photosensor is driven to read out a signal. The drive noise components are noise caused by manufacture variations such as element shapes and smear caused by element isolation and depending on radiation amounts.
The dark signal is defined as a dark current of a photosensor and greatly depends on accumulation time and temperature of the photosensor.
This drive noise will be described in detail. Variations in drive capacity of a drive element for driving a photoelectric transducer element and variations in capacity of a photoelectric transducer element cause variations in the leakage component of drive pulses. These variation components, as an information signal, are superposed on a necessary photoelectric transducer signal and are read out. The cause of generation of drive noise will be described below.
FIG. 1C is a schematic view of a photoelectric transducer element described in Japanese Patent Laid-Open Gazette No. 12764/1985, FIG. 1D is a timing chart of drive pulses for driving the photoelectric transducer element shown in FIG. 1C, and FIG. 1E is a chart showing the base potential of the photoelectric transducer element.
Referring to FIG. 1C, the photoelectric transducer element includes a base accumulation type bipolar transistor B, a drive capacitor Cox for reverse- or forward-biasing the transistor B in response to a drive pulse .phi.r, and a refresh transistor Qr. The transistor B has junction capacitances Cbc and Cbe. It should be noted that Cox, Cbc, and Cbe are referred to as capacitances or capacitors hereinafter, as needed. The capacitances Cox, Cbc, and Cbe are added to obtain a charge storage capacitance Ctot.
The operation of the photoelectric transducer element will be described below.
Assume that the initial value of a base potential VB is given as V0. When the drive pulse .phi.r is set at a potential V.phi.r at time t1, a voltage Va is applied to the base of the transistor B through the drive capacitor Cox. In this case, the voltage Va can be represented as follows: ##EQU1##
When the drive pulse .phi.rh is set at a high potential at time t2, a transistor Qr is turned on.
When the transistor B is forward-biased, the base potential VB is abruptly decreased. A time interval TC between time t2 and time t3 is a so-called refresh time interval.
The drive pulse .phi.r is set at zero at time t3, and a voltage -Va is added to the base voltage VB, so that the base voltage VB is set at V2. This reverse-biased state is the accumulation state.
The above description was confined to one photoelectric transducer element. However, a line or area sensor has a large number of photoelectric transducer elements. The capacitances of the capacitors Cox, Cbc, and Cbe between a large number of photoelectric transducer elements vary by a few fractions of 1%. For example, if the following conditions are given: EQU Cox=Cbc=Cbe.perspectiveto.0.014 pF, and V.phi.r=5 V
and the capacitance variation is 0.2%, then a variation .DELTA.Va in capacitance division voltage Va is about 3 mV.
The variation .DELTA.Va can be reduced by refreshing. However, when the refresh mode is changed to an accumulation operation mode (time t3), the variation occurs again to produce .DELTA.Vb. The variation .DELTA.Vb does not satisfy relation .DELTA.Vb=-.DELTA.Va, and the correlation cannot be established therebetween according to test results.
The above fact is assumed to be derived from different bias voltage dependencies of Cbc anc Cbe.
In the next read cycle, when the transistor B is forward-biased, the variation in base potential thereof is approximated as follows: EQU .DELTA.V.sup.2 .perspectiveto..DELTA.Va.sup.2 +.DELTA.Vb.sup.2 +2K.DELTA.Va.DELTA.b (2)
for K is -1 or more. As a result, the variation .DELTA.V becomes steady drive noise of about 4 to 5 mV.
The variation in leakage component of such a drive pulse (to be referred to as drive noise hereinafter) is eliminated according to the following conventional technique. That is, the above drive noise is stored in a memory means and is read out and subtracted from the signal read out from the sensor to obtain a true information signal.
The conventional drive noise correction technique described above requires a bulky, expensive photoelectric transducer element which does not have any industrial advantage.
In particular, in case the numbers of elements arranged in the horizontal direction and vertical direction are five hundred respectively, an area sensor requires 250,000 photoelectric elements arranged in a matrix form. In addition, when the resolution of the sensor is also taken into consideration, a memory of several megabits is required.
The unnecessary signals such as drive noise and a dark signal pose serious problems when an image of a dark object is to be picked up, i.e., image pickup at a low intensity. In the low-intensity image pickup mode, an information signal level is low and accordingly the S/N ratio is degraded. As a result, image quality is degraded. In order to improve image quality, the unnecessary signals must be reduced.
As described above, however, the dark signal primarily depends on temperature and charge accumulation time, although the drive noise rarely depends thereon. If these unnecessary signals are to be eliminated, the dark signal must be separated from the drive noise and a correction coefficient must be determined, thus requiring a large-capacity memory. As a result, signal processing is complicated and expensive, and an image pickup apparatus is undesirably bulky.